240 convertibles were converted from coupes by ASC in very limited numbers, so many coupe interior components will work. Some things, like the seatbelt pieces, obviously are convertible only but for the most part coupe stuff should work just fine.
Check out the 240 convertible forum:
zeroforum?id=148
And specifically check out he FAQ sticky in there:
zerothread/199318
Has lots of info on interior upgrades and mods for the 'vert.